1. Field of the Invention
The present invention relates to an automatic transmission to be used with a fluid coupling such as a torque converter and, more particularly, to a structure of an automatic transmission to be used in a transverse engine vehicle.
2. Related Art
Examples of automatic transmissions designed for use in a transverse engine vehicle are disclosed in Japanese Patent Publication No. 47939/1988 and in Japanese Patent Laid-Opens Nos. 141345/1987, 141342/1987 and 35904/1990.
The transmission disclosed in Japanese patent publication No. 47939/1988 includes a front planetary gear unit and a rear planetary gear unit, all aligned with an engine crankshaft. A first shaft aligned with the crankshaft at its rear end carries a counter drive gear. A second shaft, in parallel with the first shaft, carries a planetary gear unit for an underdrive and a counter driven gear at its rear end. A third shaft, in the form of axles extending from a differential unit is arranged to form a triangle, together with the first shaft and the second shaft, when viewed on end.
In the latter three mentioned publications a first shaft is arranged as output of a torque converter, with a main transmission and a counter drive gear mounted thereon. A second shaft carries a planetary gear unit for an underdrive and a counter driven gear; and a third shaft is connected to a differential unit. The counter drive gear is mounted at the central portion of the first shaft and, at the front end portion, i.e., near the torque converter, are arranged a brake unit including a brake for the main transmission, a one-way clutch unit including a 2nd coast brake (B.sub.1), a 2nd brake (B.sub.2) and a first one-way clutch (F.sub.1).
An automatic transmission (A/T) for a transverse engine vehicle is difficult to mount because it is mounted together with the engine in the restricted space under the bonnet. As shown in FIG. 10(a), a first shaft 1 aligned with the crankshaft of an engine (E/G) extends from a torque converter, through a main transmission and so on. This requires the first shaft to be so long that it can interfere with front wheel 4, restricting its steering angle. Accordingly, the first shaft should be of such a length as to leave a clearance C for the front wheel to allow a predetermined steering angle.
As shown in FIG. 10(b), the second shaft 2 which carries a planetary gear unit for the underdrive is positioned at the highest position of a triangle which is formed by itself together with the first shaft 1 and a third shaft 3. The tendency in recent years is to enhance the rigidity of the vehicle by arranging a side member 6 at an upper side portion of the bonnet in addition to a sub-frame 5 arranged in the lower portion and forming part of the chassis. Therefore, the second shaft 2 should be spaced from side member 6 to provide a predetermined clearance D to avoid interference with the side member 6.
Moreover, the third shaft 3 extends from the differential unit through a universal joint to support left-hand and right-hand wheels. Since this universal joint has its strength reduced if its bending angle becomes large, it dictates the position of the differential unit widthwise of the vehicle.
In the aforementioned automatic transmission of Japanese patent publication No. 4739/1988, the first shaft carries the counter drive gear at its rear end so that the counter driven gear on the second shaft is coplanar with the drive gear. Because the second shaft must therefore be as long as the first shaft, it interferes with the side member 6. This arrangement makes it difficult to mount the automatic transmission on the vehicle, with enhancement of the rigidity of the vehicle by the side member.
In the aforementioned automatic transmission of Japanese publications 141345/1987, 141342/1987 and 35904/1990, on the other hand, the counter drive gear is arranged generally centrally of the first shaft. This allows the drive gear to be arranged at the front end side of the second shaft so that the second shaft can be made shorter than the first shaft. However, the brake unit is arranged at the front end side of the counter drive gear of the first shaft so that the counter drive gear on the second shaft must be displaced from the center. As a result, the spacing of the second shaft portion from the first shaft portion may not be sufficient to provide for the clearance D, depending upon the position, size and so on of the side member.
Moreover, because the counter driven gear is arranged at the central portion of the second shaft, the reduction gear adjacent to the driven gear is also centrally located. On the other hand, the differential unit arranged on the third shaft has its widthwise position with respect to the vehicle body dictated by the aforementioned bending angle of the universal joint. As a result, the ring gear fixed on the differential unit and meshing with the reduction gear overhangs the differential unit in one direction, as in the automatic transmission of Japanese patent publication 47939/1988. Hence, the ring gear has its meshing plane (plane of rotation) transversely extending through the bearings supporting the differential unit. As a result, the ring gear meshing with the reduction gear with a high radial force exerts a high biasing force upon the bearings supporting the differential unit, adversely affecting the durability and reliability of the bearings.